The use of sodium thiopental, an ultrashort acting barbiturate, in cardiac surgical patients after aortic cross clamp is quite common. We have evaluated 16 patients by designing a study to quantitate drug levels in plasma during the hypothermic state in cardiopulmonary bypass procedure, in order to define effects of hyperthermia and non-pulsatile perfusion on thiopental kinetics. We have determined the actual levels of the anesthetic at the reinstitution of coronary perfusion. We have employed a simple high performance liquid chromatographic procedure for quantitation of thiopental from plasma collected from these patients. Apart from significant variability in thiopental levels, it appears that a redistribution of the drug occurs at the time of reperfusion which may determine the efficacy of the anesthetic dose. We are proceeding with analysis of data which suggests that we need to study another 5-10 patients.